SUMMARY Seasonal influenza viruses cause >600 million cases and up to 650 thousand deaths, annually. Moreover, the yearly economic losses associated with these infections run into hundreds of billions of dollars. A greater threat to human health and our economy are pandemic and avian influenza viruses, which cause severe disease, organ failure and death by dysregulating the innate immune response. Current evidence suggests that in both seasonal, pandemic and avian influenza virus infections, active viral RNA synthesis plays a critical role in triggering and dysregulating this response. The key function of viral RNA synthesis is transcription and replication of the viral genome. However, the process also produces shorter RNA products, of which the function is not fully understood. We recently discovered that pandemic and avian influenza A virus infections produce RNA molecules of about 56-125 nucleotides in length, called mini viral RNAs, and that their synthesis is correlated with the upregulation of disease markers. The underlying mechanism is that these mini viral RNAs are bound by cellular pathogen receptor RIG-I and trigger strong innate immune responses in human cells. Mini viral RNAs are produced at high levels by pandemic and avian influenza viruses, low levels by seasonal influenza virus strains, and at low levels by lab-adapted influenza virus strains. Currently, the role of mini viral RNAs in the virus replication cycle is unclear. Although mini viral RNAs induce strong innate immune responses, the pandemic and avian viruses that produce them are not impaired in their fitness, suggesting that mini viral RNAs may provide a selective advantage during outbreaks and/or that they are part of a regulatory mechanism in virus replication and growth. We will here test the hypothesis that mini viral RNAs play a key role in the viral infection cycle. We will characterise their function in viral replication and host and viral gene expression. In addition, we will investigate how the production of mini viral peptides, encoded by mini viral mRNAs, modulates host responses. In summary, this project will answer fundamental questions about the role of a new type of viral RNA in infections with highly pathogenic influenza viruses and it will contribute to a complete mechanistic understanding of influenza disease.